In computer science, high speed digital printing presents unique requirements to data-processing equipment. To operate a printing apparatus which is designed to output over 100 page-size images per minute, the ability to make the desired image data available to printing hardware requires very close tolerances in the management of the "overhead" when data is transferred from memory to printing hardware. A typical 600 spi letter-size page image, in a format suitable to be submitted to printing hardware, is typically of a size of about 4 MB; when the printing hardware demands the image data to print the particular page, this 4 MB of image data must be accessed from real memory within a time frame of approximately 300 milliseconds.
It has been found, with standard implementation of the UNIX operating system which is common in the art, that the various protocols and other steps necessary to locate and initiate transfer of image data from a real memory, and in particular from a virtual address within a real memory, often require more time than is available once the image data is demanded by the hardware. If the image data does not reach the hardware in time, typically a blank sheet will be output from the hardware, which in turn will interfere with a large printing job. There is therefore a need for a technique by which a standard operating system, such as UNIX, can be specially adapted for real-time access of image data located at a virtual address in real memory, so that the image data can be made available to printing hardware precisely when it is needed.